In a carburetor used in the internal combustion engine of a motorcycle or an automobile, there is typically provided a heating device for enhancing the atomization of the fuel and the mixing thereof with air in a regular fashion in a cold starting situation. Generally, the heating device is installed adjacent the throttle valve of the carburetor, and is operational at an ambient temperature lower than a prescribed temperature. Under these conditions, the resistance heater is made electrically conductive and the carburetor is warmed by the resistance heating, thereby melting or preventing the icing inside the carburetor. A PTC (Positive Temperature Coefficient) element has mostly been used as the resistance heater element for these devices.
The PTC element has such characteristics that, at the time when the temperature reaches a prescribed predetermined level, there is a sudden increase in the resistance value of the element. This increase in resistance will restrict the electric current passing through the PTC heater; and accordingly, automatically restrict the heating provided by the PTC element. If the PTC element continues to be electrically conductive, however, the electric power is not only wasted but there also develops a danger that the fuel will be overheated as the heat is accumulated inside the carburetor. Because of this, it is ordinarily the case that the electrical flow through the element is terminated at the time when the temperature of the carburetor has reached a suitable value.
In the heating device, according to the prior art, a bimetal type thermostat is electrically connected in series with the PTC heating element to disconnect it from the electrical source at the time when the ambient temperature (carburetor or atmospheric temperature) happens to be higher than a prescribed level. That is, the bimetal of the thermostat snaps to an open position, thereby terminating the electrical current to the PTC element.
The heating device coupled with a bimetal type thermostat, as described above according to prior art, has had restrictions in terms of the freedom of the package design, durability and reliability, switching characteristics and temperature control characteristics, etc. Irrespective of whether the bimetal type thermostat is accommodated in a separate package or in the same package with the resistance heating element, the shape and size of the bimetal has served as a bottleneck for the free design of a compact package.
Further, at the time when ambient temperature is extremely low, the item heated (carburetor) tends to be cool, with a result that the current is passed to the resistance heater on an intermittent basis. This means in the case of a bimetal type thermostat that the mechanical contacts open and close frequently, with a consequence that there is excessive contact wear and, thus, greater possibility for contact welding.
Yet further, there is a substantial hysteresis between the action or set temperature at the time when the bimetal snaps from the original position (ON position) to the responding position (OFF position); and the action temperature at the time when the bimetal returns from the responding position resulting in the fact that it has not been possible to effect the ON/OFF of electric conductivity at the same action temperature. Still further, it has been difficult to match the temperature and electric current characteristics of the bimetal with those of the PTC resistance heater; and thus, it has not been possible to freely select the temperature control characteristics of the heating device as desired.